The present invention relates to a medical appliance, and, more particularly, to a medical appliance for bridging and stabilizing spaced apart bone segments. Even more specifically, the present invention relates to a bone screw locking system for use with a medical appliance.
While the present invention is intended for use with vertebrae, it should be understood that any spaced apart bone segment may utilize the present invention.
Treatment of a herniated disk in the neck and in the lumbar region continues to be a challenging field of medicine. The classical treatment for a ruptured disk continues to be removal of the disk from between the vertebrae. By this process of removing the disk, overall spinal instability is increased. This may aggravate the patient to some degree after the operation. Another procedure previously employed is to replace the disk space with a bone graft, bringing about fusion of the vertebrae above and below the disk, eliminating the empty space between the vertebrae and improving stability.
Theoretically a diskectomy with fusion is a satisfactory procedure, though not ideal because the replaced bone does not have the principal functions of the cartilage tissue of the disk. This fusion procedure is technically demanding and has medical complications because of several physiological factors.
It must be remembered that the disk primarily serves as a mechanical cushion while permitting limited mobility. For any replacement system for a disk to be truly effective, it must allow for mobility within the natural limits of the original disk. In other words, the replacement should match appropriate joint rheology (movement behavior). The natural disk allows about 11 degrees of flexion-extension, limited lateral bending of 3 to 5 degrees, and very restricted rotation of about 1 degree. Further, any replacement system must be affixed to existing good, healthy bone in a manner that is not likely to loosen. An otherwise successful therapy is of little value if the appliance becomes loosened after the operation.
Various prosthetic devices and implants are disclosed in the art, but all are characterized by compromises to the full functions of a natural disk discussed above. Examples of the prior art include the following U.S. Pat. Nos.: 5,893,890; 5,693,100; 5,658,336; 5,653,761; 5,653,762; 5,390,683; 5,171,278; and 5,123,926. The specification and drawings of U.S. Pat. No. 6,402,755 are herein incorporated by reference.
The present invention provides a bone screw locking system for use in stabilization of spaced apart bone segments while still allowing some flexion and rearward extension of the bones with some lateral displacement. The present invention is particularly useful to stabilize adjacent vertebrae in the human spine. Again, the unique screw locking system of the present invention may be used in a medical appliance which requires joining or holding two workpieces or two separate members together.